The present invention relates generally to conductor configurations useful for generating magnetic fields having various properties and more particularly to conductor configurations useful for storing magnetic fields which have reduced mechanical forces and give rise to reduced magnetic pollution.
The electrical power industry is subject to a diurnal variation of approximately 25% in power demand. Any generating and distribution capacity, however, must consider daily peak rather than average usage. Load-leveling mechanisms have been suggested which involve storing energy from the grid during slack hours and delivering this energy back into the system during the peak hours. However, such a system must be sufficiently cost-effective as to overcome the capital investment for construction and the operational losses. One such mechanism is Superconducting Magnetic Energy Storage (SMES). Although SMES studies utilizing conventional superconducting materials have been underway for some period, the recent advancement in the area of ceramic materials which become superconducting at temperatures at or above 77K has caused increased interest by the power utilities in SMES technology.
In March of 1987, Bechtel National, Inc., released a study entitled "Superconducting Magnetic Energy Storage for Electrical Utility Load Leveling: A Study of Cost vs. Stored Energy," which focused on the ring configuration shown in FIG. 1 thereof. In the device shown, electrical current is generated in 556 composite aluminum/superconducting coil elements, each of which carries 200,000 A and stores 5000 MWh of energy. The construction cost of such a device was estimated to be about 800 million dollars. The coil would be 500 m in radius and the 10 G safety fence would be located at 2610 m radius. The magnetic field induced stress on the ring, or hoop stress, is borne by placing the entire structure in a supporting trench. One purpose of the Bechtel design was to adjust the design parameters to reduce this stress such that it could be accommodated by soil rather than by rock. Although the resulting coil would be physically larger, the increased flexibility in location of the coil along with some savings resulting from the less expensive trenching operation in soil as opposed to rock, somewhat mitigated the size consideration. Nevertheless, even under the predicted reduced stress conditions, the magnetic force at the superconducting outer rim would be 0.5 MPa or about 5.2 tons/ft.sup.2 at full coil storage. Still another difficulty with this design is the severe magnetic pollution predicted to exist at large distances from the ring structure. Such magnetic fields would have a significant affect on magnetic compass navigation instrumentation in airplanes, and perhaps have environmental side effects, such as affecting the migratory patterns of birds and dolphins.
Two publications addressing the subject of locating stationary states of flowing systems with sources having localized logarithmic potentials describe solutions to that problem which are mathematically equivalent in some instances to that of finding low force configurations of electric current carrying wires. In "Stationary Configurations Of Point Vortices And Other Logarithmic Objects In Two Dimensions," by L. J: Campbell and James B. Kadtke, Phys. Rev. Letters 58, 670 (1987), and in "Method For Finding Stationary States Of Point Vortices," by James B. Kadtke, and L. J. Campbell, Phys. Rev. A 36, 4360 (1987), the authors show equilibrium (force-free) configurations. Such configurations would yield zero magnetic forces if the vortices depicted therein were replaced by straight wires having electric current flow in the directions indicated. However, there is no teaching or identification by the authors of any relationship between the abstract formalism presented and low magnetic force obtainable in certain electric conductor configurations. In fact, the problems specifically addressed are vortex lines, line charges, and parallel screw dislocations.
Accordingly, it is an object of the present invention to identify conductor configurations having low net force on the conductors for storing energy in the form of magnetic fields.
Another object of the present invention is to identify conductor configurations having low net force on the conductors for storing energy in the form of magnetic fields where the magnetic flux pollution to the environment is minimized.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.